Superconductivity is generally defined as the property of a material to have zero resistance to electric current. Until recently only a few materials were known to have superconducting properties at temperatures above about 23.degree. K. These materials require the use of liquid helium to attain the critical temperature. Recently, higher temperature superconducting materials have been produced from ceramic materials. The ceramic materials may be fabricated in the form of a film by evaporation or by sputtering methods under vacuum. These high temperature superconducting materials have a superconductive transition temperature above 77.degree. K. thereby allowing liquid nitrogen to be used as the cryogenic material.
The high temperature superconducting materials generally consist of metal oxides bonded together to form a ceramic-like structure. In one method of production, the metal oxides are mixed together as solids and heated at sintering temperatures of 700.degree. C. to 1100.degree. C. The sintered material is then reground and reheated. The material is pressed into pellets and sintered at high temperatures for several hours. The pellets may be annealed in an oxygen environment at a lower temperature between 400.degree. C. and 800.degree. C. These ceramics tend to be brittle and fragile and cannot be drawn out to form wires or tapes. Other methods of producing superconductors include the formation of various precursors in the form of powders. The powders may be formed by forming aerosol droplets of an aqueous solution of metal compounds. The droplets are heated in a carrier gas by passing the droplets through a reactor to remove the water vapor and to convert the metal compounds to metal oxides. The furnace temperatures are typically 600.degree. C.-1000.degree. C. The droplets have a residence time in the reactor of about 15 to 56 seconds. The resulting powders are then collected on high efficiency ceramic or glass fibers.
Superconductors from Bi-Sr-Ca-Cu-O have been produced which possess superconducting phases exhibiting a critical temperature of 110K. The nominal composition has been found to have a final ratio of 2:2:2:3 which has a transition temperature of about 110K. Lead (Pb) doping of the Bi-Sr-Ca-Cu-O composition has been shown to be beneficial in promoting the formation of the 110K phase. It has been observed that as much as 50% of the initial quantity of lead is vaporized during heat processing which produces the nominal 2223 composition. The volatile nature of PbO has made it difficult to control the quantity of lead in the final composition and thus difficult to produce the desired superconducting phase.
The superconductors have been prepared by a number of different methods including the powder-in-tube method. This method typically forms a superconducting material and pulverizes the superconducting material to a powder. The superconducting material is usually produced by a solid state method which forms a non-homogeneous material. The powder is then placed in a metal tube and drawn to form a wire. The wire typically requires heating at temperatures of 800.degree. C. for 8-20 hours. This process requires long heating times which limits the production output.
These processes all have the disadvantage of producing superconducting materials either in small yields or of unpredictable composition. Furthermore, these processes use precursors that are not chemically homogeneous and thus require long heating times at high temperatures thereby inhibiting the development of a continuous process of forming superconductors. There is, therefore, a continuing need in the industry for a process of producing superconductor precursors with reduced Pb loss during synthesis and good chemical homogeneity for use in producing superconductor wires, tapes and multifilamentary articles.